The present invention relates to a cathode ray tube, and particularly to a cathode ray tube including a stem having a plurality of stem pins annularly arrayed, and sealed and extending therethrough and having glass mounds raised and surrounding the base of each of the stem pins on the side thereof supporting an electron gun, and a neck portion at one end thereof heat-sealed by the stem.
Generally, a color cathode ray tube known as a cathode ray tube emitting a plurality of electron beams has a vacuum envelope composed of a panel portion having a phosphor screen coated with a phosphor on the inner surface thereof and suspending a shadow mask therein closely spaced from the phosphor screen, a funnel portion connected to and tapered down from the panel portion, a neck portion connected, thereto, and a stem supporting an electron gun housed in the neck portion and fused to an open end of the neck portion.
The stem has a plurality of stem pins annularly arrayed and sealed and extending therethrough to support electrodes of the electron gun and introduce various signal voltages from an external circuit.
A deflection yoke is mounted exteriorly in a transition region between the funnel portion and the neck portion, and a plurality of electron beams modulated by video signals and emitted from the electron gun are deflected in two directions, i.e. horizontally and vertically to thereby reproduce a visible image on the phosphor screen.
FIG. 7 is a schematic sectional view for explaining the schematic construction of a color cathode ray tube to which the present invention is applied. Reference numeral 13 designates stem pins, 14 a stem, 20 a panel portion, 21 a neck portion, 22 a funnel portion, 23 a phosphor screen, 24 a shadow mask, 25 a mask frame, 26 a magnetic shield, 27 a shadow mask suspension mechanism, 28 an electron gun, 29, a deflection yoke, and 30 an external magnetic adjustment device.
As described above, the color cathode ray tube of this kind has a vacuum envelope compressing the panel portion 20, the neck portion 21, and the funnel portion 22 for connecting the panel portion 20 and the neck portion 21.
The panel portion 20 is formed with the phosphor screen 23 coated with three-color phosphors in the inner surface, the electron gun 28 for emitting three electron beams in a line is housed in the neck portion 21, and the shadow mask 24 having a multiplicity of apertures or a parallel array of narrow strips is arranged in the vicinity of the phosphor screen of the panel portion 20.
The deflection yoke 29 is mounted exteriorly in the transition region between the funnel portion 22 and the neck portion 21.
The electron gun 28 is housed in the neck portion 21, and the stem 14 has a plurality of stem pins 13 annularly arrayed and sealed and extending therethrough to support electrodes of the electron gun and introduce various signal voltages from an external circuit.
Three electron beams (Bc, Bs.times.2) modulated by video signals and emitted from the electron gun are deflected in two directions, i.e. horizontally and vertically by horizontal and vertical deflection magnetic fields generated by the deflection yoke 29, and are subjected to color selection at apertures in the shadow mask 24 to impinge on the respective phosphors thereby forming a color image.
FIG. 8 is a side view for explaining one example of an electron gun for a cathode ray tube. Reference numeral 1 designates a cathode, 2 a first grid electrode, 3 a second grid electrode, 4 a third grid electrode, 5 a fourth grid cathode, 6 a fifth grid electrode, 7 a sixth grid electrode, 7a a shield cup, and 8 glass rods for holding the electrodes in position (beading glass). Reference numeral 13 designates stem pins, and 14 a stem.
In FIG. 8, controlling and pre-focusing of electron beams are carried out by the first grid electrode 2 to the fourth grid electrode 5. The fifth grid electrode 6 as a focus electrode and the sixth grid electrode 7 as an anode constitute a main lens.
The shield cup 7a is connected to the anode 7 (the sixth grid electrode) and serves as an electrode part for fixing contact springs for centering the electron gun in the neck portion and for supporting a getter.
These electrodes are mounted on the stem by being welded directly or through connecting leads, to the plurality of stem pins 13 which are annularly arrayed on the stem 14, sealed thereto and extending therethrough, then the electrodes are inserted into the neck portion from its open end, and the periphery of the stem 14 is fused and fixed to the end of the neck.
FIG. 9 is an explanatory view of the operation for fusing and sealing the stem to the open end of the neck portion of the cathode ray tube. Reference numeral 10 designates an exhaust tubulation, 13 stem pins, 13a inner portions of the stem pins projecting into the interior of the neck portion, 14 a stem, and 21 a neck portion. An electron gun 28 is indicated by dotted lines.
As shown in FIG. 9, the stem 14 for supporting the electron gun 28 at its inner portions 13a projecting into the interior of the neck portion is formed of glass material, and is in the form of a flat disk whose outside diameter is smaller than that of the neck portion 21.
Stem mounds 11 are formed at the bases of the inner portions of the stem pins by raising the glass material of the stem 14 to mechanically support the inner portions 13a of the stem pins and to prevent the loss of the vacuum.
After the electron gun 28 has been secured to the stem 14, the electron gun is inserted from the open end of the neck portion 21 as indicated by the arrows, the outer peripheral portion of the stem is placed in contact with the open end of the neck portion, and the neck portion and the stem at their junctions are fused and sealed by heating them from outside the outer periphery thereof by a burner or the like.
After the stem is fused and sealed to the neck portion 21 and the vacuum envelope is evacuated to a desired vacuum with the exhaust tubulation 10 connected to a vacuum pumping system, the exhaust tubulation 10 is sealed off.
FIG. 10 is a sectional view of main parts for explaining a fused portion of the neck portion and the stem in sealing a conventional cathode ray tube. Reference numeral 12 designates a groove, and 15 a fused portion. The same reference numerals are used in FIG. 10 as used for corresponding parts in FIGS. 1 to 9.
As explained in FIG. 9, in sealing the cathode ray tube, the stem 14 is placed in contact with the open end of the neck portion 21, their junctions are heated and fused by using a burner or the like, the force is applied so that the stem 14 is somewhat pulled away from the neck portion 21 to thereby form a constricted portion in the fused portion 15 as shown in FIG. 10.
In the aforementioned conventional cathode ray tube sealed off by fusing the stem 14, a sufficiently large difference (R1-R2) between a distance (hereinafter also called an inner radius) R1 from the tube axis to the inner wall in a region of the neck portion housing a major portion of the electron gun 28 supported on the inner portions 13a of the stem pins projecting into the interior of the neck portion, and a distance R2 from the tube axis to the outside edge of the stem mounds 11 measured at half the axial height H of the stem mounds 11, has been secured to thereby prevent the occurrence of crack in a fused portion of the stem.
Therefore, for example, a stem having a diameter of 15.24 mm of a circular array of stem mounds 11 (corresponding to a diameter of a stem pin circle) has been coupled to a neck portion having an outside diameter of 29.1 mm and an inside diameter of 23.9 mm. In this case, the minimum value of R1 is approximately 12 mm and the maximum value of R2 is approximately 9.3 mm, and the difference (R1-R2) is nearly equal to 2.7 mm.
There has been a problem in that it is difficult to fuse and seal such a large-pin-circle stem to a neck portion of, for example, a so-called narrow neck low-deflection-power cathode ray tube having a neck narrower than a presently widely used neck having a nominal diameter of 29.1 mm.
There has been another problem in that, since a connecting portion between glass of the stem 14 and glass of the neck portion is very close to the groove 12 formed in the fused portion, crack tends to occur in that portion.